In 2012, the Minnesota Legislature amended the solid waste definition in state statute to exempt slurry resulting from concrete cutting in the construction, improvement or repair of roads (Minn. Stat. § 116.06, subd. 22). The exemption changes acceptable methods of disposal for concrete slurry and allows it to be disposed on site. The slurry is only exempted providing certain best management practices (BMPs) are utilized when managing disposal of the slurry at the road project site. This guidance describes the BMPs required to manage the slurry to comply with the statute and agency rules to protect water quality.
Liquid wash water from concrete-truck washing at construction sites is not included with the slurry definition and is required to be managed according to MPCA’s NPDES/SDS Construction Stormwater General Permit requirements:
Why BMPs are needed for concrete slurry management
Concrete slurry is generated when activities such as concrete diamond grinding or saw cutting occur as associated with the construction, repair or improvement of roads.
While hardened concrete is relatively benign, liquefaction of concrete products via diamond grinding and saw cutting may carry by-products, including silica, cadmium and other pollutants to receiving waters in slurry or in sediment form.
Concrete slurry can be highly alkaline and may leach into the ground and contaminate groundwater. If it were to migrate into area lakes or streams via storm drains or overland flow, it could raise the pH of these waters and harm aquatic life.
Solids that are improperly disposed of can clog storm drain pipes and cause flooding.
The suspended solids in the slurry will ultimately become sediment after evaporation of the slurry water. If not properly disposed of, these sediments can migrate to waters of the state due to stormwater runoff during and post construction.
Sediment in surface water reduces water clarity making the water body unsuitable for recreation or for desirable fish populations. The suspended sediment in the water column interferes with plant photosynthesis, animal respiration and the ability of fish to feed properly. Deposited sediment on a stream bottom eliminates suitable spawning habitat for many organisms. Once settled, the contaminated concrete sediment may act as a reservoir for particulate forms of pollutants, such as heavy metals, that may continue to be released into the water over time.
Therefore, proper containment, recovery and disposal of concrete slurry not only helps prevent pollution but is also a matter of good housekeeping at a construction site.
Applicable MPCA regulations
Concrete slurry resulting from concrete cutting in the construction, improvement or repair of roads is not a solid waste as defined by Minn. Stat. Section 116.02, subd. 22 when deposited per the provisions in Minn. Stat. Section 161.376. Concrete slurry shall be treated as a solid waste in all other cases.
State Water Quality Rule
Minn. Rule 7050.0210, Subp. 2: Nuisance conditions prohibited. No sewage, industrial waste, or other wastes shall be discharged from either point or nonpoint sources into any waters of the state so as to cause any nuisance conditions, such as the presence of significant amounts of floating solids, scum, visible oil film, excessive suspended solids, material discoloration, obnoxious odors, gas ebullition, deleterious sludge deposits, undesirable slimes or fungus growths, aquatic habitat degradation, excessive growths of aquatic plants, or other offensive or harmful effects.
At no time during the disposal of concrete slurry will the owner or operator be allowed to violate the above cited state water quality rule. Best management practices (discussed below) must be implemented, maintained, and modified as needed to prevent the creation of nuisance conditions in waters of the state.
Best management practices
Slurry spreading disposal methods
In rural areas that have vegetated slopes the slurry can be deposited on the in-slopes as the grinding operation progresses down the road.
The in-slopes must be 3:1 or flatter, should have a recognizable upland back-slope, and the slurry must be deposited at a rate which will not allow it to reach the wetted perimeter of the highway ditch – as per design.
In some cases, areas without a recognizable upland back-slope may also be suitable.
In-slope disposal areas must be reviewed by the owner and operator (contractor) and agreed upon as suitable prior to disposal to ensure the slurry does not descend into the wetted perimeter. Additional monitoring may be necessary in some areas.
As part of the contract documents (and/or SWPPP), the owner and operator shall identify wetlands and other sensitive areas where slurry discharge operations are not permitted. Spreading disposal is prohibited in areas of environmental sensitivity. Discharge from bridge decks or into an enclosed storm sewer system is prohibited.
The owner or operator shall make a site inspection prior to the start of grinding or cutting operations to identify areas of environmental sensitivity. Lack of a recognizable upland back-slope may indicate that the highway borders a wetland, lake or stream. Also check project documentation for areas of environmental sensitivity, such as native vegetation, etc.
The slurry generated while grinding in unpermitted areas shall be picked up and hauled for disposal in non-sensitive areas on the job.
The slurry shall not be allowed to flow across the roadway into adjacent lanes.
The diamond grinding equipment shall be equipped with a well-maintained vacuum system that is capable of removing all standing slurry, leaving the roadway in a damp condition after the grinder passes.
The vacuumed material shall be spread evenly (not allowed to reach the wetted perimeter of the ditch) on the adjacent in-slopes by dragging a flexible hose with a spray bar or other approved device along the slope.
All areas where disposal is prohibited shall be clearly marked.
The spreading should not take place on the shoulder.
Spreading should begin a minimum of 1 foot from the shoulder, with each pass of the grinder moving the spreading operation farther down the in-slope to ensure no build-up of grinding residue.
Spreading shall be restricted to areas above the wetted perimeter of ditches and in a manner that is in compliance with Minnesota Rules, part 7050.0210.
At no time will the grinding residue be allowed to enter a drainage system [wetted perimeter of ditch]. The contractor is responsible for providing suitable means (e.g., perimeter control via compost log) to prevent the discharge of the grinding residue into the drainage system.
Slurry collection and pond decanting
The slurry can be collected in water-tight haul units and transported to lined settlement ponds constructed by the owner or operator. These cannot be permanent treatment ponds.
These ponds may be constructed within or outside the right-of-way. All locations shall be approved by the engineer.
These ponds shall be constructed to allow for the settlement of the solids and decanting of the water for reuse in the grinding operation.
At the completion of the grinding operation, the remaining water will be allowed to evaporate.
After drying, the remaining solids may be used as a fill material, a component in recycled aggregate or any other commercially useful application. Up to 0.5 cubic yards of concrete slurry solids may be managed on-site. If concrete slurry solids are buried on site, they should be at least two feet below the surface and must not be buried within 3 feet of the groundwater table. Quantities larger than 0.5 cubic yards of concrete slurry solids must either be managed with the rest of the site’s solid wastes or obtain an approval from the MPCA’s solid waste program for other beneficial use options.
The pond area shall be reclaimed to its original condition and vegetated to protect against erosion.
Slurry collection and plant processing
The slurry shall be collected and hauled as with pond decanting. There are various plant designs that may be used such as centrifuge and belt press. The plant site shall be prepared to control any storm water runoff in accordance with state regulations.
The site should be restored and vegetated at the completion of operations. The processed water and solids are to be handled in the same fashion as on site slurry collection and pond decanting. The site may be within or outside the right-of-way. Site locations are to be approved by the engineer.
pH control plan
With either the spreading or pickup operation, the owner or operator will monitor and control the pH of the slurry. The slurry must be managed to maintain a pH below 12 and greater than 6, to ensure the establishment/maintenance of vegetative cover and slope stability.
At the start-up of operations, test the pH at least once per hour to ensure it is within the acceptable limits. The test equipment should be calibrated daily and approved by the engineer. Once the pH control plan is operational and producing consistent results, the testing frequency may be reduced to 4 tests per day.
The owner or operator must log all test results and deliver a signed copy to the engineer weekly. Slurry containing a pH outside the above limits cannot be deposited on the ground. The owner or operator must determine how to maintain the slurry within the acceptable range and get approval from the engineer.
In addition to state requirements, there may be city, county or watershed management organization requirements that may be more stringent than those found in this guidance.